(1) Field
The disclosed methods and systems relate generally to multiple cell batteries and more particularly to methods and systems for connecting battery cells.
(2) Description of Relevant Art
There are a number of applications for batteries that cannot be served by off-the-shelf battery cell sizes as these applications may require a battery cell that is larger or of a different geometry than the available standard battery cell sizes. Designing and manufacturing a new battery cell can be expensive due to significant design and tooling costs. Additional costs include testing and evaluating the new battery cell's performance. The reliability of custom battery cells can be a concern because the battery cells are typically manually assembled, and may not have a long-term operational history and proving period. Some battery chemistries, such as Lithium-ion (Li-ion), have a flammable electrolyte, and can pose a safety hazard if the battery cell is overcharged or otherwise abused. Making the battery cells in very large sizes places a larger amount of potential energy in one battery cell, making the larger battery cell a higher risk than the smaller, commercial battery cells.
A number of high volume manufacturers manufacture small battery cells in standard sizes. For example, a 18650 battery cell format is a common battery cell used in laptop computers, cellular phones, and other small portable equipment. The 18650 battery cell format is available for the Nickel metal-hydride and Lithium-ion chemistries. It is estimated that there are over 700 million Lithium-ion battery cells made annually in the 18650 battery cell format. High volume, automated manufacturing, and competition help to maintain the 18650 battery cells at cost competitive prices and high quality. The small battery cells can also be considered as generally safe, since only a limited amount of energy is stored in a single volume, as opposed to the larger battery cells.
It is generally believed that parallel operation of battery cells is undesirable since the cells may not share the total battery current evenly, thereby increasing the possibility of overheating and damaging individual battery cells. A short circuit in one battery cell could be supplied by other battery cells that are connected in parallel with it, potentially causing a catastrophic failure. It is also generally believed in the industry that increasing the number of battery cells in a battery pack can be detrimental to battery pack reliability because of an increase in overall parts count.
One prior art system discloses an architecture that includes connecting battery cells in parallel, where each battery cell has over-current and over-temperature protection. A disadvantage of this system is the inability of the architecture to tolerate a battery cell with a high resistance short, sometimes referred to as a “soft” short. In this type of failure, a battery cell loses its ability to maintain charge during extended standing periods. In a parallel arrangement, the battery cell with the soft short not only dissipates its own charge, but also dissipates the charges of the battery cells connected in parallel with the soft short cell. The current flow due to the soft short may not be large enough to activate the over-current mechanisms described in the prior art system, nor does such soft short condition produce enough heat to activate an over-temperature mechanism.
Another possible failure mode in a prior art system is a short circuit current that can activate a battery cell's Polymeric Positive Temperature Coefficient (PPTC) circuit protection device, but not a series fuse. The PPTC is a common component in lithium-ion battery cells that requires a small amount of heating current while the short is present, and drains the battery cells of their energy while the battery is in an idle state. In the prior art systems, the occurrence of a soft short may completely disable the battery pack over an extended time.